Alkaline phosphatase concentrations usually increase in blood neutrophils of normal pregnant women. However, the maternal neutrophil alkaline phosphatase (NAP) in Down's syndrome pregnancies (women with trisomy 21 fetuses) differs from NAP found in normal pregnancies. NAP from women with Down's syndrome pregnancies is characterized by: (1) an increase in NAP enzyme activity over that found in normal pregnancies, (2) NAP thermal stability, (2) NAP stability in urea, (3) a significant decrease in reactivity with anti-liver-type alkaline phosphatase (AP); (4) low reactivity with anti-placental-type AP or anti-intestinal-type AP antibodies; (5) altered response to AP enzyme inhibitors; and (6) marked dispersion of NAP lead citrate reaction products or anti-NAP antibody colloidal gold-labeling in neutrophil cytoplasm, as detected by electron microscopy. These characteristics suggest that neutrophils of a woman with a trisomy 21 fetus contain two AP isoenzymes: the liver/bone type AP and an atypical AP that is related to the early placental form. Thus, the non-specific alkaline phosphatase isoenzyme can be an "enzyme marker" to diagnose Down's syndrome pregnancies.
The histochemical measurement of maternal NAP has a high detection rate for the prenatal detection of Down's syndrome pregnancies. However, because the histochemical method is laborious and subjective to use, the method has not gained widespread acceptance in prenatal screening programs.
Some automated methods have been developed. Tafas et al. have developed an image analysis method for the measurement of neutrophil alkaline phosphatase (NAP) and established a correlation of urea-resistant fraction of NAP (URNAP)/NAP scoring between the manual and automated methods for prenatal screening of Down's syndrome (U.S. Pat. No. 5,352,613 to Tafas et al.; Tafas et al., Fetal Diagn. Ther. 11(4):254-259, 1996). Measurements ("scores") obtained by manual and automated methods correlate, but the automated scoring is threefold faster. However, a less laborious and subjective automated image analysis method could have benefits in the medical arts.